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  <title>Document</title>
  <script>
    function backDay () {
      var year = prompt('请输入年份：');
      if (judge(year)) {
        alert('当前年份是闰年2月份有29天');
      } else {
        alert('当前年份是平年2月份有28天');
      }
    }
    backDay();

    // 判断是否为闰年的函数
    function judge (year) {
      var flag = false;
      if (year % 400 == 0 || year % 4 == 0 && year % 100 != 0) {
        flag = true;
      }
      return flag;
    }
  </script>
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</body>

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